GaN system compound semiconductors (InxGayAl1-x-yN, 0≦x, y; x+y≦1) such as GaN, InGaN, AlGaN, InGaAlN and the like have been expected as materials of semiconductor electronic devices such as a light emitting device, a power device and the like and have been remarked as materials applicable in other various fields.
In an earlier development, it is difficult to grow a bulk crystal of the GaN system compound semiconductor. Therefore, a substrate obtained, for example, by forming a thin film single crystal such as GaN or the like on a different type crystal such as a sapphire or the like by hetero epitaxy has been used for the electronic device.
However, because lattice mismatching between a sapphire crystal and a GaN system compound semiconductor crystal is large, a dislocation density of the GaN system compound semiconductor crystal grown on the sapphire crystal becomes large. Therefore, a problem has been arisen that crystal defects are generated. Further, a sapphire has a low thermal conductivity not to easily release heat. Therefore, when a substrate having a GaN system compound semiconductor crystal grown on a sapphire crystal is used for an electronic device or the like largely consuming electric power, a problem has been arisen that the electronic device easily becomes a high temperature.
Further, the growth of a GaN system compound semiconductor crystal according to an epitaxial lateral overgrowth (ELO) method or the like using a hydride vapor phase epitaxial growth (hereinafter, abbreviated as HVPE) method has been tried. The ELO method is, for example, a method of forming an insulating film acting as a mask on a sapphire substrate, forming an opening in a portion of the insulating film to use the insulating film as the mask and growing a GaN system compound semiconductor crystal having high crystallinity while using an exposed surface of the sapphire substrate as a seed of the epitaxial growth.
In this method, the growth of the GaN system compound semiconductor crystal starts from the surface of the sapphire substrate placed on the inside of the opening formed in the mask, and a grown layer is spread on the mask. Accordingly, the dislocation density in the crystal can be suppressed to a low value, and a GaN system compound semiconductor crystal having small crystal defects can be obtained.
However, the GaN system compound semiconductor crystal obtained by the ELO method has large thermal distortion. Therefore, when the polishing is performed for the GaN crystal after the growth of the GaN crystal to separate the sapphire substrate from the GaN system compound semiconductor crystal inn order to obtain a GaN system compound semiconductor crystal wafer as a single substance, a problem has been arisen that the wafer is bended due to residual distortion.
Therefore, the inventors have proposed a method of using a rare earth 13 (3B) group perovskite crystal as one of materials of a different type crystal substrate and growing a GaN system compound semiconductor according to a hetero epitaxy while setting a {011} plane or a {101} plane of the perovskite crystal as a growth plane (No. WO95/27815). The {011} plane and the {101} plane denote a set of planes equivalent to a (011) plane and a set of planes equivalent to a (101) plane respectively.
In this growth technique of the above-described previous application, when GaN is, for example, grown on a {011} plane or a {101} plane of a substrate while using NdGaO3 (hereinafter, abbreviated as NGO), which is one of the rare earth 13 (3B) group perovskites, as the substrate, the lattice mismatching is almost 1.2%. Therefore, the lattice mismatching can be considerably lowered as compared with a case where a sapphire or SiC used in place of the sapphire is used as a substrate. Accordingly, because the dislocation density in the crystal is lowered, a GaN system compound semiconductor crystal having small crystal defects can be grown.
Further, the invention obtained by modifying the technique of the previous application has been proposed (Japanese Patent Application Publication No. Tokkai 2000-4045). In this invention, a GaN thin film layer is formed on an NGO substrate at a low temperature (400 to 750° C.), heat treatment is performed for the GaN thin film layer to heat up the GaN thin film layer to a predetermined temperature in an inert gas (N2 gas) atmosphere, and thereafter a GaN thick film layer is grown at a high temperature (800 to 1200° C.) on the GaN thin film layer. In this technique, because the GaN thin film layer is formed as a buffer layer before the formation of the GaN thick film layer, the buffer layer can prevent the NGO substrate from being reduced by reacting with NH3 or the like at a growth temperature (800 to 1200° C.) of the GaN system compound semiconductor. Accordingly, the deterioration of a grown GaN system compound semiconductor crystal caused by the reduction of the NGO substrate can be avoided.
However, in the technique of the above-described previous application (Japanese Patent Application Publication No. Tokkai 2000-4045), it was found out that a problem was arisen that the crystal quality is degraded, because the GaN thin film layer is formed at the temperature (400 to 750° C.) considerably lower than the growth temperature (800 to 1200° C.) of the GaN thick film layer in order not to reduce the NGO substrate. Further, the GaN thin film layer (a lower temperature buffer layer) having inferior crystal quality is required to be thickly formed to prevent the NGO substrate from being reduced during the heating up to the growth temperature of the GaN thick film layer. Therefore, it was found out that a problem was arisen that the crystallinity is further degraded.
Therefore, in the method described above, though the NGO substrate can be prevented from being reduced at the growth temperature of the GaN thick film layer, it is assumed that there is a probability that the GaN thin film layer adversely influences the GaN system compound semiconductor crystal grown on the GaN thin film layer so as to lower the crystal quality of the GaN system compound semiconductor crystal.
An object of the present invention is to provide a technique where, in a GaN system compound semiconductor crystal production method using a substrate (for example, a rare earth 13 (3B) group perovskite such as NGO or the like) capable of reacting with a raw material or being degraded due to heat during the rearing of a GaN system compound semiconductor layer, the crystal quality of the GaN system compound semiconductor crystal can be enhanced by improving the quality of a GaN thin film layer formed between the substrate and a GaN system thick film layer.